1. Field of the Invention
This invention relates to cellular telephone systems and, more particularly, to processes for modeling code division multiple access (CDMA) cellular telephone systems.
2. History of the Prior Art
Presently available commercial mobile communication systems typically include a plurality of fixed base stations (cells) each of which transmits signals to and receives signals from mobile units within its communication area. Each base station in a CDMA system communicates with mobile units by sending digital transmissions over the same frequency spectrum. In most cellular systems, especially those with cells in urban areas carrying heavy traffic, each base station may be further divided into two or three sectors each of which may include its own transmission equipment the antennas of which provide 180 or 120 degree coverage, respectively. When the term base station is used herein, both sectors and cells are intended unless the context indicates otherwise.
A CDMA system transmits messages digitally. All transmissions in a CDMA system are on the same frequency spectrum so the digital signals constituting each message must somehow be recognizable from all of the possible transmissions available. To accomplish this, the digital messages are encoded by a series of overlaid digital codes. One of these codes, called a pseudorandom noise (PN) code, is applied to all transmissions throughout a CDMA system. The PN code is used to encode the individual bits of the original message at the transmitter and to decode the encoded message at the receiver. In order to recognize messages from a particular base station, each base station uses a distinct time offset (called a PN offset) from some repeating initial time to begin encoding the transmission using the PN code. Thus, one base station may begin an encoded transmission at the initial time, a second base station at an offset of one unit from the initial time, a third at an offset of two units from the initial time, and so on up to a total of 512 offsets.
Each transmission between a mobile unit and a base station is also placed on what is effectively a separate channel by further encoding the transmission with one of a plurality of Walsh codes. A message encoded using a Walsh code, as with a PN code, can only be decoded by the same Walsh code at the receiver. Thus, an encoded transmission on a particular channel is decoded by applying a mask including both the Walsh and PN codes to the received pattern of information bits commencing at the PN offset designated for the particular base station.
A base station normally has sixty-four Walsh codes available for defining channels on which it can establish transmissions with mobile units. Certain of these channels are preassigned to function as control channels. For example, in order to advise mobile units of the particular PN offset used, each base station continuously broadcasts the PN code using its assigned PN offset on one of these channels (a pilot channel) defined by Walsh codes. Mobile units monitor this preassigned pilot channel. When a mobile unit finds an offset at which a pilot is decodable, it refers to another control channel (a synchronization channel) to determine the initial time and thereby identify the PN offset of the base station. Each system also maintains a paging channel upon which indications are posted that new messages are arriving. A total of nine channels are provided for these and other control functions.
In order to allow mobile units to transmit and receive telephone communications as the units travel over a wide geographic area, each base station is normally physically positioned so that its area of coverage is adjacent to and overlaps the areas of coverage of a number of other base stations. When a mobile unit moves from an area covered by one base station to an area covered by another base station, communication with the mobile unit is transferred (handed off) from one base station to another base station in an area where the coverage from different base stations overlaps.
In most other types of cellular communication systems, a mobile unit communicates with only one base station at a time. However, since all transmissions in a CDMA system take place on the same frequency spectrum, a mobile unit actually has available all of the information which is within its range. However, it only decodes information on PN offsets and Walsh code channels which are directed to it. A CDMA mobile unit uses a receiver which is able to apply a number of decoding masks simultaneously at different offsets of the entire spectrum of information which it receives. At present, a mobile receiver may decode as many as six PN offsets at once. However, usually only three PN offsets are used to decode messages while the others decode control information. Because a mobile unit in a CDMA system may be receiving the same information from a number of different base stations at the same instant, it may decode information from a single message sent to it from a number of different base stations simultaneously using different PN offsets and Walsh codes and combine that information to produce a single output message. Thus, while a signal transmitted from one base station may be fading, the same message may be being received with adequate strength from another base station. This allows a CDMA system to offer the possibility of significantly better transmission. The situation in which a mobile is communicating with a number of base stations at once is called xe2x80x9csoft handoff.xe2x80x9d
In order for a system operator to allocate resources to a cellular telephone system intelligently, the operator typically models the system. In order to utilize the advantages offered by CDMA technology, an operator should be able to model the system accurately. However, because a CDMA system can involve a plurality of base stations each communicating with the same mobile unit simultaneously, much more data must be dealt with and more resources have been required than are available to accurately model such a system. This has led to the use of probability techniques (typically Monte Carlo techniques) which utilize only a small number of locations throughout the system and extrapolate between those locations. This has produced less than accurate results leading to incorrect allocation of assets.
It is desirable to provide a new process by which the properties of a CDMA cellular system may be modeled accurately so that steps may be taken to improve the system.
The present invention is realized by a computer implemented process which utilizes actual received signal level data gathered from closely spaced locations covering an entire CDMA system to calculate interference levels at each location, determines those base stations which most probably communicate with a mobile unit at each particular location, compiles a list of neighbor base stations for each base station at each location throughout the system, and determines the transmit power necessary for each base station to communicate with a mobile unit at each location, calculates interference levels at each base station, and determines the transmit power necessary for a mobile unit at each location to communicate with each base station which most probably communicates with a mobile unit at that particular location.
These and other features of the invention will be better understood by reference to the detailed description which follows taken together with the drawings in which like elements are referred to by like designations throughout the several views.